Cathode structure with magnetic field producing means

ABSTRACT

A cathode assembly and an electrical circuit that includes a conductor physically parallel to and electrically in series with a filament so that when the circuit is energized the magnetic fields produced by the current flowing in the filament and the conductor oppose each other, thereby decreasing the dispersing effect of the filament magnetic field on a portion of the electrons emitted from the filament. The diameter of the conductor is sufficiently larger than the diameter of the filament so that only the filament emits electrons when the circuit is energized.

2,1 17,709 5 /1938 Dayisson United States Patent William H. Shriner Blanchester, Ohio 855,888

Sept. 8, 1969 Aug. 31, 1971 The Bendix Corporation [72] Inventor [21 Appl. No. [22] Filed [45] Patented [73] Assignee [54] CATHODE STRUCTURE WITH MAGNETIC FIELD PRODUCING MEANS 7 Claims, 3 Drawing Figs.

[52] U.S. (I 313/63, 250/419 SB, 313/154, 313/155, 313/230 [51] Int. Cl l-l0lj 3/32 [50] Field of Search 313/63,

230,154, 155, 84, 333; 250/419 SB, 41.9 SE, 41.9 SA, 41.9 TF; 315/14 [56] References Cited UNITED STATES PATENTS 1,940,185 12/1933 Reagan....

2,533,767 12/1950 Coeterier 313/155 X 3,115,591 12/1963 Brunnee 250/4119 X 3,172,007 3/1965 Hanks et al. 315/14 Primary Examiner-Roy Lake Assistant Examiner- Palmer C. Demeo Anorneys Raymond J. Eifier and Flame, Arens, Hartz, Hix &

Smith ABSTRACT: A cathode assembly and an electrical circuit that includes a conductor physically parallel to and electrically in series with a filament so that when the circuit is energized the magnetic fields produced by the current flowing in the filament and the conductor oppose each other, thereby decreasing the dispersing effect of the filament magnetic field on a portion of the electrons emitted from the filament. The diameter of the conductor is sufficiently larger than the diameter of the filament so that only the filament emits electrons when the circuit is energized. I

PATENTEI] AUGBI I971 3,602,752

sum 1 0r 2 FlGURE 2 FIGURE l WILLIAM H. JHRINER INVENTUR.

'A TTORIYVEY PATENTED M1831 l9?! 3, 6 02,752

321m 2 SF 2 FIGURE 3 WILLIAM H. JHRINER INVENTOR.

wmwyd zw AYTORNEY CATIIODE STRUCTURE WITH MAGNETIC FIELD PRODUCING MEANS BACKGROUND OF INVENTION This invention relates to mass spectrometers and more particularly to ion sources having a cathode assembly wherein the effect of the magnetic field of the filament on electrons is greatly reduced.

The mass spectrometer is an instrument that permits rapid analysis of molecular species by measurement of the masses of the different ions after ionization of the molecules. In operation, a small amount of gas to be analyzed is admitted through a sample inlet into an ionization chamber where the gas is ionized by electrons emitted from a filament. The ions are then directed by an electric field from the ionization chamber and into a region where the ions are separated according to their mass to charge ratio (m/e). The ions then impinge upon the cathode of an electron multiplier to achieve a gain of or greater. The resulting output signal is then coupled to a device which indicates the mass spectrum of the particular gas. The spectrum indicates the elements and/or molecules which make up the gas. For accurate analysis i.e.quantitative and qualitative, the ions in the ionization chamber should be formed into a compact pack or group of ions so that there is distinct resolution between ions of difierent masses at the spectrometer output. To produce an ideal ion pack, the electrons emitted from the filament in the ionization chamber should be a closely spaced pack of electrons having the smallest possible cross section so that when they collide with the gas molecules a uniform and compact pack of ions is formed. To produce this type of electron and ion pack filaments were made of wire having a diameter of 0.01 inch or less. It has been discovered by the inventor that wire filaments set up a magnetic field which disperses the electrons leaving the filament rather than concentrating the electrons in a compact pack. This increases the size (cross section) of the electron pack which increases the size of the ion pack which results in poor resolution between spectrum lines at the spectrometer output.

BRIEF SUMMARY OF INVENTION To produce a compact pack or group of ions for distinct separation according to their mass to charge ratio (m/e) the cathode filament of an ionization chamber is magnetically clamped to reduce the spreading of ion forming electrons. The invention is characterized by a conductor which sets up a magnetic field opposing the direction of the magnetic field established by current flowing through the filament so that a portion of the electrons emitted from the filament are subjected to a magnetic field having an intensity less than the magnetic field intensity set up by current flowing through the filament. This reduces the dispersing effect of the filament magnetic field on a portion of the electrons emitted from the filament (i.e. those electrons accelerated from the filament to the collector) and hence reduces the spreading of electrons. The conductor is further characterized by the fact that it has a diameter at least 2 times greater than the diameter of the filament and is in closely spaced relationship with the filament, preferably parallel.

Accordingly, it is an object of this invention to reduce the dispersing effect of the magnetic field set up by current flowing through a filament on a portion of the electrons emitted from the filament.

It is an object of this invention to combine in the same electric circuit a filament and conductor in such a manner that when a potential is applied to the circuit the electrons emitted from a portion of the filament will be subjected to a resulting magnetic field which is less than the magnetic field set up by current flowing through the filament.

It is an object of this invention to have two current carrying conductors, one of-which is a filament, arranged so that a portion of the electrons emitted from the filament are subjected to a magnetic field having a direction different then a magnetic field established by current flowing through the filament so that dispersal of electrons is reduced.

It is another object of this invention to produce a dense pack of electrons having a narrow cross section.

It is still another object of this invention to improve the output of an ion source.

It is a further object of this invention to improve the resolution of a mass spectrometer.

It is still a further object of this invention to provide a cathode assembly with a magnetically damped cathode filament.

The above and other objects and features of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings and claims which form a part of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a preferred embodiment of a magnetically damped cathode filament.

FIG. 2 shows a mass spectrometer utilizing the magnetically damped cathode filament.

FIG. 3 shows an ion source utilizing the advantages of magnetically damped cathode filaments.

DESCRIPTIONOF THE PREFERRED EMBODIMENT Referring now to the drawings, FIG. 1 is a preferred embodiment of a cathode assembly 50 utilizing the principles of the invention. Filament 5 is mounted on supports 51, 52 which are mounted on a base 53. Conductor 6 is mounted on the supports 51, 52 and is electrically in series with filament 5 via conducting paths 61 and 62. Power supply 55 supplies the electric potentials through leads 56, 57 which causes current to flow through the conductor 6 and filament 5. Once a current is established in the circuit, filament 5 emits electrons by thermionic emission. The combination may be modified or changed for adaptation to different systems but important to achieving the objects of the invention is the fact the current flowing through the conductor 6 flow in a direction opposite of the current flowing through the filament 5. This changes the direction of the magnetic field established by current flowing through the filament and reduces the dispersing effect that the filament magnetic field has on a portion of the emitted elec trons. The conductor 6 and filament 5 should be in closely spaced relationship, approximately 0.250 inches or less, and preferably parallel to each other. The materials which comprise the conductor and filament are chosen so that only the filament emits electrons. To prevent the conductor 6 from emitting electrons by thermionic emission it is preferred that the diameter of the conductor be at least twice the diameter of the filament 5.

FIG. 2 shows a mass spectrometer of the type described in US. Pat. Nos. 2,765,408 and 2,685,035. Molecular species entering the ionizing region 1 from the sample inlet 3 are ionized by electrons from the filament 5 of a cathode assembly described in FIG. 1. The effect of the magnetic field set up by current flowing through the filament 5 on the electrons has been reduced by the current flowing through the conductor 6 and, therefore, the scattering of electrons is reduced. This results in a denser packet of ions than would be obtained if the electrons were scattered by the undampened magnetic field of the filament 6. The packet of ions 10 are then into the drift tube by accelerating grids 9. Because of the length of the drift tube 7, the ions, which are traveling at different velocities, are separated according to their mass to charge ratio (m/e) striking the cathode 21 at different times. The different times of flight (T) for ions of different masses (m) to travel from the ionizing region to the cathode may be calculated mathematically from the equation T=k(m/e)", where k is a constant depending on the physical dimensions of the system. A magnetic electron multiplier 30 (e.g. US. Pat. Nos. 2,983,845 and 2,743,370) is used to detect and amplify the ion bunches. The

ions strike the cathode and produce secondary electrons 40. The electrons then follow a cycloidal path under the influence of the mutually perpendicular electric and magnetic fields in the multiplier to strike the dynode strip 31 multiplying in number to achieve a gain of approximately 10. The resulting output signal is then synchronized on an oscilloscope, or gated (gates 35) to an analog (not Shown) for strip chart recording.

FIG. 3 shows an exploded view of an ion source similar to that disclosed in U.S. Pat. 2,743,370. A filament 5 made from a suitable material is provided. The filament 5 is arranged in parallel relationship with a conductor 6 which is electrically in series with the filament. The cathode assembly 50 is the same or similar to the cathode assembly described in H6. 1 which produces a closely spaced pack of electrons. A control electrode 70 is disposed at a relatively close distance to the filament 5 and is provided with a slot 7] aligned with the longitudinal axis of the filament 5. An accelerating electrode 72 having a slot 73 is similarly spaced unaligned. Similarly a collimating electrode 74 having a slot 75 is disposed adjacent the accelerating electrode 72. A collector 76 is disposed in a substantially parallel relationship to the collimating electrode 74 on the far side of the ionization chamber 80. The ionization chamber 80, preferably having 2 sides 82, 86 mounted in spaced relationship by posts 87, has at least one side which permits the introduction of a to be analyzed. Side 82 performs the function of an ion draw-out grid and has a wire mesh 83 covering a slot 84 therein. A conduit 91 introduces gas from cylinder 90 into the ionizing chamber 80. A power supply 95 supplies the proper operating voltages to the cathode, electrodes and collector so that when a gas is introduced into the ionization chamber 80, electrons emitted from filament 5 are accelerated through electrodes 70, 72, 74 and into the chamber 80 where they collide with gas molecules to form ions. The electrons passing through the chamber 80 are collected by the collector 76.

OPERATION Referring to FIGS. 1, 2, 3, the cathode assembly performs as follows. When the cathode circuit is energized (FIG. I) a current flows through the conductor 6 and the filament 5 which emits electrons by thermionic emission. Since the current flowing through the conductor 6 is the opposite direction of the current flowing through the filament the magnetic fields set up by the current in the filament and conductor are in opposite direction i.e., clockwise and counterclockwise. This has the effect of increasing the magnetic field intensity between the conductor and the filament and decreasing the magnetic field intensity on the side of the filament 5 away from the conductor. Therefore, the opposing magnetic fields subject a portion of the electrons emitted from the filament (the side away from the conductor) to a magnetic field which has an intensity less than the magnetic field intensity set up by current flow through the filament (the resulting field established by the opposing magnetic fields of the conductor and filament). This reduces the dispersing effect that the filament's magnetic field has on electrons emitted from the side of the filament away from the conductor. As a result of confining the electrons to a smaller volume electron density is increased. This increases the density of the ions and reduces the size of the ion packet necessary to obtain a representative sample of the gas to be analyzed. Finally, the reduced ion packet improves the resolution of mass spectrum lines at the spectrometer output.

While a preferred embodiment of the invention has been disclosed it will be apparent to those skilled in the art that changes may be made to the invention as set forth in the appended claims, and, in some cases, certain features of the invention may be used to advantage without corresponding use of other features. For example, although the filament 5 is referred to as electrically in series with the conductor 6 separate circuits may be used to produce currents through them having opposite directions. Also the filament and conductor may be arranged so that the conductor is part of the means that supports the filament. Accordingly, it is intended that the illustrative and descriptive materials herein be used to illustrate the principles of the invention and not to limit the scope thereof.

Having described the invention what is claimed is:

l. A cathode assembly which comprises:

a straight wire filament for emitting electrons;

a means for supporting said filament; and

input means for receiving an electric potential to be applied to said filament, said input means including an electrical conductor having a straight portion with a diameter greater than said straight wire filament adjacent to and in parallel relationship with said filament, said electrical conductor being in electric and magnetic circuit relationship with said filament so that when an electric potential is applied to said input means an electric current flows through said filament and said electrical conductor to produce opposing magnetic fields whereby a portion of the electrons emitted from said filament are subjected to a magnetic field having an intensity less than the magnetic field intensity set up by current flowing through said filament.

2. A cathode assembly as recited in claim 1 wherein the spacing between said filament and said conductor is less than 0.250 inches.

3. A cathode assembly as recited in claim I wherein the ratio l/ 2 is greater than 2 where D is the diameter of the conductor and D is the diameter of the filament.

4. A cathode assembly as recited in claim 2 wherein the ratio l 2 is greater than 2, where D is the diameter of the conductor and D is the diameter of the filament.

5. A cathode assembly which comprises:

a filament for emitting electrons;

a means for supporting said filament; and

input means for receiving an electrical potential to be applied to said filament, said input means including an electrical conductor having a straight portion adjacent to and in parallel relationship with said filament, said electrical conductor being in electrical and magnetic circuit relationship with said filament so that when an electric potential is applied to said input means an electric current flows through said filament causing it to emit electrons and an electric current flows through said straight portion of said electrical conductor, which does not emit electrons, to set up a resulting magnetic field having a direction which reduces electron dispersal of a portion of the electrons emitted from said filament.

6. In combination with an ion source of the type having means for accelerating electrons in a given direction, means for receiving molecules of gas and directing said molecules into said electrons to cause ionization of said molecules and means for accelerating and directing said ions along a predetermined path, the improvement which comprises:

a cathode assembly having a filament for producing electrons;

a means for supporting said filament; and

input means for receiving an electric potential to be applied to said filament, said input means including an electrical conductor in electric and magnetic circuit relationship with said filament so that when an electric potential is applied to said input means an electric current flows in said filament and said electrical conductor to produce opposing magnetic fields whereby a portion of the electrons emitted from said filament are subjected to a magnetic field having an intensity less than the magnetic field intensity set up by current flowing through said filament.

7. In combination with a mass spectrometer of the type wherein a small amount of gas to be analyzed is ionized by conductor in electric and magnetic circuit relationship with said filament so that when an electric potential is ap plied to said input means an electric current flows in said filament and said electrical conductor to produce opposing magnetic fields whereby a portion of the electrons emitted from said filament are subjected to a magnetic field having an intensity less than the magnetic field set up by current flowing through said filament. 

1. A cathode assembly which comprises: a straight wire filament for emitting electrons; a means for supporting said filament; and input means for receiving an electric potential to be applied to said filament, said input means including an electrical conductor having a straight portion with a diameter greater than said straight wire filament adjacent to and in parallel relationship with said filament, said electrical conductor being in electric and magnetic circuit relationship with said filament so that when an electric potential is applied to said input means an electric current flows through said filament and said electrical conductor to produce opposing magnetic fields whereby a portion of the electrons emitted from said filament are subjected to a magnetic field having an intensity less than the magnetic field intensity set up by current flowing through said filament.
 2. A cathode assembly as recited in claim 1 wherein the spacing between said filament and said conductor is less than 0.250 inches.
 3. A cathode assembly as recited in claim 1 wherein the ratio D1/D2 is greater than 2 where D1 is the diameter of the conductor and D2 is the diameter of the filament.
 4. A cathode assembly as recited in claim 2 wherein the ratio D1/D2 is greater than 2, where D1 is the diameter of the conductor and D2 is the diameter of the filament.
 5. A cathode assembly which comprises: a filament for emitting electrons; a means for supporting said filament; and input means for receiving an electrical potential to be applied to said filament, said input means including an electrical conductor having a straight portion adjacent to and in parallel relationship with said filament, said electrical conductor being in electrical and magnetic circuit relationship with said filament so that when an electric potential is applied to said input means an electric current flows through said filament causing it to emit electrons and an electric current flows through said straight portion of said electrical conductor, which does not emit electrons, to set up a resulting magnetic field having a dIrection which reduces electron dispersal of a portion of the electrons emitted from said filament.
 6. In combination with an ion source of the type having means for accelerating electrons in a given direction, means for receiving molecules of gas and directing said molecules into said electrons to cause ionization of said molecules and means for accelerating and directing said ions along a predetermined path, the improvement which comprises: a cathode assembly having a filament for producing electrons; a means for supporting said filament; and input means for receiving an electric potential to be applied to said filament, said input means including an electrical conductor in electric and magnetic circuit relationship with said filament so that when an electric potential is applied to said input means an electric current flows in said filament and said electrical conductor to produce opposing magnetic fields whereby a portion of the electrons emitted from said filament are subjected to a magnetic field having an intensity less than the magnetic field intensity set up by current flowing through said filament.
 7. In combination with a mass spectrometer of the type wherein a small amount of gas to be analyzed is ionized by electrons in an ionizing region and wherein an electric field directs said ions out of said ionizing region into a region wherein said ions are sorted according to their ratios of mass to charge, the improvement which comprises: a cathode assembly having a filament for producing electrons; a means for supporting said filament; and input means for receiving an electric potential to be applied to said filament, said input means including an electrical conductor in electric and magnetic circuit relationship with said filament so that when an electric potential is applied to said input means an electric current flows in said filament and said electrical conductor to produce opposing magnetic fields whereby a portion of the electrons emitted from said filament are subjected to a magnetic field having an intensity less than the magnetic field set up by current flowing through said filament. 